A method and an apparatus for encoding/decoding a digital signal are provided. First, a digital input signal is transformed into samples to remove redundant information among signals. Then, a lookup table corresponding to a characteristic of the input signal is selected among a plurality of lookup tables that indicate different numbers of bits allocated for each quantization unit depending on different characteristics of input signals, and the number of bits allocated for each quantization unit is acquired from the selected lookup table. Next, a distribution of samples within each quantization unit is divided into a predetermined number of sections, and the samples are linearly quantized using the allocated number of bits on a section-by-section basis. Thereafter, a bitstream comprised of frames is produced from the quantized samples and predetermined side information so that information about a frame length is stored in the end of frame.